The edge structure of power semiconductor components is accorded a particular importance since the blocking capability of real pn junctions is drastically reduced compared with the bulk blocking capability of a plane-parallel junction because a major curvature of the equipotential lines and thus a significant increase in field strength occur in the edge region of the semiconductor component. The critical field strength in silicon is thus attained substantially earlier. Without additional measures, the blocking capability in the case of a power semiconductor component of the 1200 volts class with a 5 μm deep p-type well typically amounts to only 15% of the bulk breakdown voltage. For economic and thus practical operation of the semiconductor component, it is necessary for the dielectric strength of the edge termination to approach that of the cell array as near as possible. In order additionally to increase the robustness of the component, the blocking capability of the edge should even exceed that of the cell array. A further requirement made of an optimum edge structure is to take up little silicon area, in order to keep down the chip costs.
Known structures and measures for reducing such increases in field strength are field rings, single- or multistage field plates, less highly doped p-type zones located upstream of the p-type well, laterally varied p-type doping or combinations of these measures. It has thus been possible to increase the blocking capability up to approximately 90% of the bulk blocking capability. What is disadvantageous about all these structures and measures is that, depending on the voltage class or type of the power semiconductor component, complex simulations are necessary for the correct design of the edge termination, and, moreover, the fabrication of such edge structures requires additional method steps and additional chip area. A further disadvantage of such edge structures is the charge that is stored in the edge region in the case of bipolar power semiconductor components and increases the turn-off losses without significantly improving the properties of the semiconductor component in the on-state case.
In order to reduce the required chip area of the edge structure of a trench power semiconductor component, DE 101 27 885 A1 discloses arranging the field plate in the case of low-voltage transistors at least partly in a trench structure. With this form of edge termination, the insulation layer of the edge trenches is made thicker on the outer sidewall arranged opposite to the cell array, in order thus to increase the breakdown strength. The edge trench lined with an insulation layer is incidentically filled with conductive material, in particular polycrystalline silicon, which thus forms a field plate reaching into the depth of the component.
What is disadvantageous about this form of an edge structure is the dimensioning of the component edge that has to be determined anew for each voltage type, and the very thick insulator layers required for high reverse voltages.